In the winter of 1417, Poggio Bracciolini set out for a remote mountain-top monastery to seek out a manuscript he thought might change Western civilization. An adventurer, humanist and Papal secretary, Bracciolini was well-suited to the task. He was young – in his thirties – hardy and experienced. He had already made several such trips to European monasteries, where monks continued to write and preserve the ancient classical poems and essays. In fact, frequently these remote outposts were the last places on earth where the great texts could be found.

The question is why did the monks bother to write them, and book hunters try to find them. After all, the Greeks and Romans were pagans and most of their ideas and writings were anathema to and in some cases even proscribed by the church. As we learn in Stephen Greenblatt’s “The Swerve: How the World Became Modern,” under the 6th century Benedictine Rule, monks were sworn to the hard work of copying texts, not to preserve them but to improve their reading, which was a primary requisite for the job.

As Greenblatt writes, “All monks were expected to know how to read. In a world increasingly dominated by illiterate warlords, that expectation … was of incalculable importance.” If a monk could not read well, he was, according to the Rule, “compelled” to learn. Greenblatt continues: “It was this compulsion that, through centuries of chaos, helped to salvage the achievements of ancient thought.”

Bracciolini was an unlikely stalwart in the art of salvaging ancient texts. A commoner, he rose through the Papal ranks to become the Pope’s private secretary on the strength of his craftiness, intelligence and, of all things, beautiful handwriting. That’s hard to imagine today, when youngsters write with their thumbs and most signatures are indecipherable scrawls. But as Greenblatt points out, in a bureaucracy like the Vatican rife with texts, missives, correspondence, memos, bulls and other documents, producing handwritten copies clearly and quickly was a necessity.

Bracciolini was also a classicist – a member of the increasingly active and growing tribe of scholars who revered the ancient texts for their brilliance and wisdom. He had already succeeded in recovering several valuable manuscripts. Still, book hunters knew that many classics remained missing – perhaps waiting to be rediscovered and to bring great fame to the finder – including major works by Euripides, Aristophanes, Aeschylus and Sophocles.

One such presumably lost text was On the Nature of Things (in Latin, De Rerum Natura), a famous 7,400-line poem by the 1st century B.C. writer Lucretius. The poem’s underlying premise, based on the teachings of the 3rd century B.C. Greek philosopher Epicurus, was that all life is the result of chance and consists of atoms piercing the void, sometimes in unpredictable patterns – the swerve. Time is eternal, there is no creator and no afterlife, and all organized religions are superstitious delusions. Epicureans believed the guiding principle of life was to seek pleasure and avoid pain. No wonder it was banned with particular vehemence by the church.

As a professed Christian and high-ranking Papal functionary, Bracciolini certainly did not endorse this worldview. But as an obsessive and successful book hunter, he delighted in the often-rigorous hunt for lost texts and took pleasure and pride in finding them. When he spotted On the Nature of Things in a monastery library in central Germany, he knew he had made a major discovery.

As related by Greenblatt, professor of Humanities at Harvard and author of “Will in the World: How Shakespeare Became Shakespeare,” copies of the poem circulated slowly in intellectual circles at first and encountered considerable intellectual resistance. The underlying concepts took decades to influence society. But influence they did. Within 150 years Epicurean and Lucretian references populate Montaigne’s essays and are later reflected in the works of Spencer, Bacon and Hobbes. Isaac Newton, writing in 1718, described himself an “atomist” and endorsed many Epicurean scientific principles. And in 1776 Thomas Jefferson, an avowed Epicurean who owned numerous editions of On the Nature of Things, wrote that men are entitled not just to life and liberty but “the pursuit of happiness.”

Interspersed with Bracciolini’s story are many fascinating side trips, such as daily life in medieval monasteries; the volcanic extinction and later explorations of Pompeii and Herculaneum; the development of public libraries in ancient Rome; the origins and uses of scrolls, codices and paper; the creation of the italic font; and the gossip, rivalries, schisms and narrow-mindedness of the 15th century church.

On that last point Greenblatt comes in for some criticism. The Catholic priest and author Father Robert Barron argues passionately in his blog, Word on Fire, that in claiming the medieval church sought to strangle intellectual freedom, the book is biased and misguided. That Father Barron fails to mention the Inquisition, Galileo or the church’s Index of Prohibited Books (abolished only in 1966) does not entirely nullify his critique.

And in the nature of such things, the book’s subtitle seems considerably exaggerated and the “swerve” of the title poorly explained. But these are minor matters: the book, which won the 2012 Pulitzer Prize for non-fiction, is a joy from first swerve to last.